Inverted microtrip transmission line integrated in a multilayer structure

ABSTRACT

The invention relates to a transmission cable realized by multilayer technique, where the signal cable ( 20 ) is set at a desired distance from the wall of the cavity constructed for said transmission cable by means of a separate support element ( 25 ). The ground cable ( 21 ) included in the structure is placed on the cable cavity wall opposite to the signal cable. By using the transmission cable according to the invention, there is achieved a low attenuation per unit of length at RF frequencies.

This application claims the benefit of the earlier filed InternationalApplication No. PCT/FI00/00274, International Filing Date, Mar. 30,2000, which designated the United States of America, and whichInternational application was published under PCT Article 21(2) inEnglish as WO Publication No. WO04/62368.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a transmission cable constructed by multilayertechnique, said cable being located in a cavity with a first surface anda second surface essentially parallel to the first, said transmissioncable consisting of a signal cable that is essentially parallel with thefirst surface of the cavity, and of a ground cable that is placed onsaid second surface essentially in parallel with the signal cable.

2. Brief Description of Related Developments

Various different cable structures are utilised in the construction ofelectronic appliances. As the frequencies of operation increase, thereare higher requirements set for the cable structures to be used, inorder to prevent attenuation caused by said cable structures. Atpresent, in the structures of electronic appliances, there is generallyapplied the so-called multilayer technique, which is based either on theHTCC technique (High Temperature Cofired Ceramics) or on the LTCCtechnique (Low Temperature Cofired Ceramics). With both manufacturingmethods, the produced structures consist of several green tapes, with athickness of about 100 μm, which are positioned one on top of the other.Prior to thermal treatment, the material still is soft, so that in thegreen tapes, there can be made cavities of desired shapes. Likewise, atdesired spots, there can be silk-screened various electrically passiveelements. The elastic layers are laminated together by means ofpressure. In order to prevent the lamination pressure from collapsingthe structure that contains various cavities, the pressurising must becarried out according to a so-called unaxial method. This means that thepressure is directed to the object only in the direction of the axis zof said object. Finally the resultant structure is burnt, in the case ofLTCC at 850 degrees and in the case of HTCC at 1,600 degrees. In theelements to be produced, at the cavities there are made perforationsthrough which the excess pressure created in the burning process can belet out.

In FIGS. 1a and 1 b, there is illustrated a possible alternative forrealising an inverted microstrip cable based on the HTCC or LTCCmultilayer technique according to the description above. In a preferredembodiment, the structure according to FIG. 1a is achieved by joiningtogether, during the production process but prior to the burning step ofthe structure, the exemplary elements 12 and 13 illustrated in thedrawing. Both of said elements are made layer by layer of some suitabledielectric material in a fashion described above. In the element 13,there is machined a rectangular groove, on the bottom of which there issilk-screened a signal cable 10. The thickness 18 of the element 13, asshown in FIG. 1b, when measured at the bottom of the groove, issufficient to prevent disturbing ground potential levels from comingclose to the described inverted microstrip cable. In the exampleillustrated FIG. 1b, the angle between the side walls of the groove madein the element 13 and the groove bottom 16, 17 is 90 degrees, but inprinciple the angles can have some other size, too. On the surface ofthe element 12, there is silk-screened a ground cable 11, the widthwhereof corresponds to the width of the groove made in the element 13.The elements 12 and 13 are machined separately, and when they areconnected, there is obtained a structure according to FIG. 1a, wherethere is created a gas-filled cable cavity 14.

In FIG. 1b, there is illustrated a cross-section made in the directionA-A′ of FIG. 1a. The attenuation and impedance of a transmission cableaccording to the invention are determined by the permitivity (∈_(r)) ofthe employed elements 12 and 13, as well as the geometric shape of thegroove. From the drawing it is seen that the electromagnetic fieldemitted from the signal cable 10, said field in the drawing beingillustrated by the power lines 15, proceeds a long way inside theelement 13. With RF frequencies, the permitivity of the element 13 isclearly higher than the permitivity of the gas mixture filling the cablecavity 14. This results in that the cable attenuation is stronglyincreased with RF frequencies. The final multilayer structure of theapparatus also includes other material layers than those illustrated inFIGS. 1a and 1 b, in which layers there may be provided passivecomponents, cavities for active components and other cable structures,too.

However, the use of electric circuits manufactured by the abovedescribed techniques becomes problematic, if very high frequencies mustbe used (RF applications). Signal attenuation in a cable structurerealised with LTCC technique at the frequency of 20 GHz rises up to 0.2dB/cm, and in a cable structure realised with HTCC technique up to 0.6dB/cm. In such RF applications where low attenuation is required, forexample in filters and oscillation sources having a high quality factor(Q value), the above described techniques are no longer feasible.

Another problem with regular microstrip cables or inverted microstripcables is the impedance level of the transmission cables realised bymeans of structures. An uncontrolled fluctuation of the impedance levelresults in undesired reflections of the signal back in the directionwhere the signal came from, or in radiation in the cable surroundings.Impedance is affected by the geometric shape of the cable structure, aswell as by the relative permittivity (∈_(r)) of the surrounding materiallayers. In prior art structures, the above described two factors are theonly free choices for adjusting the impedance.

With prior art LTCC and HTCC structures, another drawback is presentedin the dispersion of the phase velocity with high frequencies. In adispersed signal, the signal components contained therein at differentfrequencies have passed through the transmission cable at differentvelocities. This phenomenon distorts the received signal, and anexcessive increase of the dispersion results in that the received signalbecomes inapplicable.

From the U.S. Pat. No. 3,904,997 there is known an arrangement where thesignal cable of an inverted microstrip resting on a substrate is encasedin a shell-like structure made of metal. By means of this arrangement,both the attenuation of the transmission cable and the stray radiationscattered from the cable are attempted to be reduced. The metallic cablecavity must always be manufactured in advance, and its fastening in areliable way to the rest of the multilayer structure causes problems.The fact that the thermal expansion coefficient of the metallic cablecavity is different from the basic substrate may cause the structure tobreak at the junction surface. Moreover, the structure includes a lot ofmanually performed work steps, wherefore it also is expensive inmanufacturing costs.

From the U.S. Pat. No. 5,105,055 there is known an arrangement where inone flexible, cable-like structure there are integrated several cables.In said structure, the signal cable is attached to a dielectricsubstrate, and the ground cable is placed in a cavity-like structuremade of another dielectric material. In principle, said cable is anentity made of several inverted microstrip cables. The materials of thecable structure are chosen among such materials that are elastic, andthey can be processed with extrusion devices designed for processingplastics. Several variations of the cable structure are presented in thepublication. According to said publication, the cable is meant to beused in connection with personal PC devices. Also in this case it ispointed out that owing to the target of usage, the materials chosen inthe structure do not enable the use of RF frequencies.

The object of the invention is to reduce the described drawbacksconnected to the prior art.

The transmission cable placed in a cavity according to the invention ischaracterised in that it comprises a support element with a surfaceessentially parallel to the first and second surfaces of the cavity,said support element being placed between said first and secondsurfaces, so that the signal cable is realised by means of anelectroconductive material layer formed on the surface of said supportelement.

A number of preferred embodiments of the invention are set forth in theindependent claims.

The basic principle of the invention is as follows: by applyingmultilayer technique, there is manufactured a modified, invertedmicrostrip cable, where the signal cable is attached, by means of aspecially designed support element, on one surface of the cable cavity.Thus the effect of the material layers that encase the cable to theelectromagnetic field surrounding said cable is remarkably reduced.

An advantage of the invention is that at RF frequencies the attenuationof a transmission cable according to the invention is clearly lower thanwith existing inverted microstrip cables, because the electromagneticfield emitted from the signal cable is mainly located in the gas-filledcable cavity, the permittivity (∈_(r)) of said cable cavity with respectto the permittivity of the surrounding dielectric materials being low.

Another advantage of the invention is that the transmission cable can befully integrated in a multilayer structure without any specific worksteps carried out expressly for this purpose.

Yet another advantage of the invention is that thereby the impedancelevel of the transmission cable can be adjusted as desired in a simplefashion.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below. The description refersto the accompanying drawings, wherein

FIG. 1a shows in a perspective illustration a prior art invertedmicrostrip cable realised by multilayer technique,

FIG. 1b shows a cross-section of the transmission cable of FIG. 1a, seenalong the line A-A′,

FIG. 2 shows in cross-section a preferred embodiment according to theinvention,

FIG. 3 shows in cross-section another preferred embodiment according tothe invention,

FIG. 4 shows in cross-section a third preferred embodiment according tothe invention,

FIG. 5 shows in cross-section a fourth preferred embodiment according tothe invention, and

FIG. 6 shows in cross-section a fourth preferred embodiment according tothe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIGS. 1a and 1 b were already dealt with in connection with thedescription of the prior art.

FIGS. 2-6 represent a few preferred embodiments according to theinvention. All embodiments illustrated in the drawings consist ofelements manufactured by multilayer technique, which elements can in themanufacturing process be combined to form a uniform structure. In thepreferred embodiment of the invention illustrated in FIG. 2, the signalcable 20 of an inverted microstrip cable is attached to a supportelement 25 according to the invention. The walls surrounding thetransmission cable can be made in a process explained above, inconnection with the description of the prior art, for instance of two ormore elements 22 and 23, which both are compiled of several green tapes.The sectional plane 26 of the elements, perpendicular to the patterns,is chosen so that the number of work steps in the manufacturing processis minimised. The support element 25 can likewise be made in severalalternative ways. For example, it can be made so that the contactsurface of the elements 22 and 23 is placed exactly on the level of thesupport element surface, which in the drawing is illustrated by a dottedline 26. On both sides of the support element 25, there are made groovesseen in the illustration.

Another alternative is to make a groove in the element 23 according tothe method described in connection with FIG. 1a and to manufacture thesupport element 25 and the signal cable 20 separately with respect tothe sectional plane, starting from the sectional plane illustrated bythe dotted line 27. The support element 25 and the signal cable 20 arein later manufacturing steps attached, as a uniform structure, on thebottom of the groove made in the element 23. The ground cable 21 is madeeither in the way described in connection with FIG. 1a, or it may besilk-screened in a groove of a suitable size provided in the element 22,if the contact surface of the elements 22 and 23 is the planeillustrated by the dotted line 26. When the elements 22, 23 and thesupport element 25 are connected, the ground cable 21 is placed in thecable cavity in parallel with the signal cable 20. From the drawing itis seen that the electromagnetic field emitted from the signal cable 20towards the ground cable 21, said field being illustrated by the powerlines 24, clearly makes a shorter passage in the dielectric material, inthe support element 25, than it has to make in the case of FIG. 1b,inside the element 13 made of a dielectric material. The major part ofthe transmission cable losses are composed exactly of the losses made inthe dielectric material layer. As a consequence, the inverted microstripcable according to the invention has a smaller attenuation per unit oflength than the inverted microstrip cable according to the prior art.However, the impedance level of the transmission cable according to theinvention can be adjusted to the desired magnitude, because theimpedance of the transmission cable is affected by adjusting the outerdimensions of the support element 25 made of some dielectric material.

In the embodiment illustrated in FIG. 3, the signal cable 30 of aninverted microstrip cable is attached to a support element 35, which isnarrowed in a triangular fashion towards the bottom of the transmissioncable cavity. The cable structure according to the drawing is composedof at least two separate elements 32 and 33. The contact surface of theelements, which in the drawing is illustrated by the dotted line 36, ischosen to be the best possible with respect to the manufacturing of thestructure. The contact surface 36 of the elements 32 and 33 can be, asis illustrated, the plane of the signal cable 30 attached to the supportelement 35, but it can also be some other plane. The support element 35can be produced in connection with the production of the element 33, butit can also be produced separately, in which case its contact surfacewith the element 33 can be a plane which in the drawing is illustratedby the dotted line 37. Part of the electromagnetic field, illustrated bythe power lines 34, emitted from the signal cable 30 towards the groundcable 31, proceeds for a short length inside the support element 35. Thepart of the electromagnetic field that is left inside the supportelement is smaller than the part left in the bottom substrate in theprior art arrangement illustrated in FIG. 1b. In the illustratedpreferred embodiment, the attenuation per unit of length is thus lowerthan the attenuation of an inverted microstrip cable according to theprior art.

In the embodiment illustrated in FIG. 4, the signal cable 40 of aninverted microstrip cable is attached to a support element 45 that iswider towards the bottom of the groove made in the element 43. Theillustrated structure is composed of at least two separate elements 42and 43. The elements are treated so that inside the elements 42 and 43,there is created a cable cavity according to the illustration. Thecontact surface of the elements 42 and 43, illustrated by the dottedline 46, is chosen to be the best possible with respect to themanufacturing of the product. The contact surface of the elements 42 and43 can be, as is illustrated, a plane of the signal cable 40 attached tothe support element 45, but it may also be another plane that isadvantageous for the manufacturing process. In this embodiment, part ofthe electromagnetic field, illustrated by the power lines 44, emittedfrom the signal cable 40 towards the ground cable 41, proceeds throughthe support element 45. However, the part that passes through thesupport element is remarkably smaller than in the case of the prior artinverted microstrip cable illustrated in FIG. 1b. Thus the attenuationper unit of length also in this embodiment is lower than in a prior artinverted microstrip cable.

In the embodiment illustrated in FIG. 5, the signal cable 50 of aninverted microstrip cable is attached to a support element 55 having theshape of a T-beam. The walls encasing the transmission cable arecomposed of at least two elements 52 and 53, and the sectional planeperpendicular to the patterns of said elements, said sectional planebeing illustrated by the dotted line 56, is chosen so that the number ofwork steps in the manufacturing process is minimized. The supportelement 55 can be manufactured in several alternative ways. Onealternative is to produce the support element 55 and the signal cable 50separately, starting from the plane at the base of the T-beam, whichplane is illustrated by the dotted line 57. The support element 55 andthe signal cable 50 are connected, as a uniform structure, to theelement 52. The ground cable 51 can be produced for instance in the wayillustrated in connection with FIG. 1b. When the elements 52, 53 and 55are connected together, the ground cable 51 is located in the cablecavity on the opposite side of the signal cable 50. In FIG. 5 it is seenthat the electromagnetic field emitted from the signal cable 50 towardsthe ground cable 51, which field in the drawing is illustrated by thepower lines 54, passes only a short way in the dielectric material, inthe support element 55. As a consequence, the inverted microstrip cableaccording to the drawing has an extremely low attenuation per lengthunit, in comparison with the attenuation of a prior art invertedmicrostrip cable.

SUMMARY OF THE INVENTION

In the embodiment illustrated in FIG. 6, the transmission cablestructure is composed of at least two elements 62 and 63. The contactsurface of the elements 62 and 63, illustrated by the dotted line 66, ischosen to be the best possible with respect to the manufacturing of theproduct. It may be located at the illustrated point, in which case it islevel with the surface of the support element 65, which in the drawingis illustrated by the dotted line 66. In this embodiment, the shape ofthe support element is inwardly curved. The support element 65constitutes part of the element 63. Also in this embodiment only a smallpart of the electric field is emitted from the signal cable 60 towardsthe ground cable 61, which in FIG. 6 is illustrated by the power lines64, proceeds in the dielectric material of the support element.Likewise, also in this embodiment the attenuation of an invertedmicrostrip cable according to the invention is low in comparison with acorresponding prior art transmission cable.

In the embodiments described above, the inverted microstrip cableaccording to the invention is placed in a cable cavity made ofdielectric material layers. The number of the layers constituting thecable cavity wall may vary according to the employed technique and to anoptimal number of working steps. The wall strength of the created cablecavity is assumed to be so good in all directions, that the other groundpotential levels possibly located in the surroundings are placed farenough in order to prevent the shape of the electromagnetic field of thetransmission cable from being disturbed thereby.

The invention is not restricted to the described embodiments only. Forexample, the structure of the walls forming the cable cavity can bedivided into various levels by innumerable different ways. The employedmanufacturing technique determines which method of dividing the wallparts to be created is optimal with respect to expenses and output.Likewise, the shape of the support element according to the inventioncan deviate from the preferred embodiments illustrated above. Also themanufacturing method of the employed signal and ground cables may beother than the suggested silk screen method. Other known cablestructures, for example coplanar cable, can also be employed as thecable used in the structure. The inventive idea can be applied invarious different ways within the scope of the patent claims.

What is claimed is:
 1. A transmission cable constructed by multilayertechnique, located in a cavity comprising a first dielectric surface anda second dielectric surface which is essentially parallel with the firstsurface, said transmission cable comprising: a signal cable, which isessentially parallel to the first cavity surface, and a ground cable,which is placed on said second surface, essentially in parallel with thesignal cable, and wherein said transmission cable also comprises adielectric support element which has a dielectric surface that isessentially parallel with said first and second surfaces and is locatedbetween said first and second surfaces, so that said signal cable isprovided with an electroconductive material layer disposed on thedielectric surface of the support element.
 2. A transmission cableaccording to claim 1, wherein the support element is rectangular inshape.
 3. A transmission cable according to claim 1, wherein the supportelement is a square.
 4. A transmission cable according to claim 1,wherein the shape of the support element is a T-beam.
 5. A transmissioncable according to claim 1, wherein the shape of the support element isa surface defined by two curved surfaces.
 6. A transmission cableaccording to claim 1, wherein the signal cable is an inverted microstripcable.
 7. The transmission cable of claim 1 wherein the first surfaceand the second surface share common side surfaces to define the cavity.8. The transmission cable of claim 1 wherein the electroconductivematerial layer is disposed along an entirety of the surface of thesupport element.
 9. The transmission cable of claim 1 wherein a plane ofcontact between the first surface and the second surface is along aplane of a surface of the support element.
 10. The transmission cable ofclaim 1 wherein a groove is disposed along each side of the supportelement, each groove being disposed between a wall of the cavity and aside of the support element.
 11. The transmission cable of claim 1wherein the signal cable and the ground cable are separated by a mediumthat decreases attenuation.
 12. The transmission cable of claim 11wherein the medium is a gas or a vacuum.
 13. A transmission cablecomprising: a first dielectric surface substantially parallel to asecond dielectric surface, which together with common, substantiallyperpendicular side walls, defines a cavity; a signal cable located nearthe first surface and essentially parallel to the first surface; aground cable on the second surface and essentially parallel to thesignal cable; and a support element having a dielectric surfacesubstantially parallel to the first and second surfaces and locatedbetween the first and second surfaces, wherein the signal cable isprovided with an electroconductive material layer disposed on anentirety of the surface of the support element, a groove is disposedalong each side of the support element, each groove being disposedbetween a wall of the cavity and a side of the support element.
 14. Atransmission cable comprising: a first dielectric surface substantiallyparallel to a second dielectric surface, which together with common,substantially perpendicular side walls, defines a cavity; a signal cablelocated near the first surface and essentially parallel to the firstsurface; a ground cable on the second surface and essentially parallelto the signal cable; and a support element having a dielectric surfacesubstantially parallel to the first and second surfaces and locatedbetween the first and second surfaces, wherein the signal cable isprovided with an electroconductive material layer disposed on anentirety of the surface of the support element.
 15. The transmissioncable of claim 14 wherein the cavity provides a medium to decreaseattenuation.
 16. The transmission cable of claim 15 wherein the mediumis gas or a vacuum.